* Pass the appropriate flags to GCC when building the Perl bindings.

  Without these, Nix fails on 32-bit Linux with Perl 5.14, with a
  rather unhelpful error message:

    Not a CODE reference at /nix/store/n6kpbacn6nn7i3i735v8j3di8aqyl07v-perl-5.14.2/lib/perl5/5.14.2/i686-linux-thread-multi/DynaLoader.pm

  This is likely because the lack of -D_FILE_OFFSET_BITS=64 causes
  various Perl structures to not match what the Perl interpreter
  expects.